1. Field of the Invention
The present invention relates to a power control method of a mobile communication system, and in particular, to a method for controlling power of a Transport Format Combination identifier (TFCI) for a Downlink Shared Channel (DSCH) based on existence of handovers, main base station, and reception of transmit power control (TPC) signals from base stations.
2. Description of the Related Art
FIG. 1 is a block diagram for illustrating soft handover between base stations located in the same radio network controller (RNC) (intra RNS, Inter Node B Soft handover) in a conventional wireless network
As shown in FIG. 1, a serving RNC (SRNC) 106, located in a universal mobile telecommunication system (UMTS) under a core network (CN) 101, controls dedicated radio resources assigned to a mobile station (UE) 110 in a serving radio network subsystem (SRNS) 104.
If the mobile station 110 moves from a service area of a base station 108 to a service area of another base station 109, both base stations 108 and 109 demodulate signals from the mobile station 110 and send the demodulated frames to the SRNC 106. The SRNC 106 can select an optimal one among the received frames. In this manner, the mobile station 110 can maintain a communication channel by communicating with two base stations 108 and 109. In this case, the SRNC 106 and the base stations 108 and 109 are included in the SRNS 104.
FIG. 2 is a block diagram illustrating a soft handover between two different radio network subsystems (RNSs) in another conventional wireless network.
As shown in FIG. 2, the mobile station 110 is able to maintain a communication channel with two base stations 109 and 116 that are located in different RNSs during the soft handover, when a serving radio network controller(SRNC) 106 and a drifting radio network controller(DRNC) 114 control a plurality of respective base stations 108, 109, 116, and 118. In this case, the SRNC 106 controls dedicated radio resources assigned to the base stations 108 and 109 located in the DRNS 112, and the DRNC 114 provides radio resources to the mobile station 110 when it moves from the DRNS 104 to the DRNS 112. The SRNC 106 and the DRNC 114 are located in the SRNS 104 and DRNS 112.
As described above, each of the RNCs 106 and 114 includes a plurality of base stations, and the mobile station 110 simultaneously maintains communications with the two base stations located within the respective RNSs 104 and 112. In this case, even though the handover occurs between two base stations, the mobile station always performs communication with at least two base stations.
The third generation partnership project (3GPP) standard specifies a downlink shared channel (DSCH) for the burst data type.
FIG. 3 shows a DSCH frame format. As shown in FIG. 3, the DSCH format has a length of 10 ms and each frame can be used by different users assigned with a channelization code.
Also, the DSCH frame having a code indicating a predetermined data rate can be assigned to just one user at any time. Accordingly, a specific mobile station occupying the DSCH performs the power control by itself.
Typically, the DSCH operates together with a Dedicated CHannel (DCH). That is, the mobile station that occupies the DSCH must have the DCH. The mobile station measures the power of DCH received from the base station and responsively generates and transmits a Transmit Power Control (TPC) signal to the base station such that the base station updates DCH power on the basis of the TPC. Also, the base station can update the DSCH power according to the updated DCH power. This kind of DCH operating with the DSCH is called as an associated DCH.
FIG. 4 shows a DCH frame format. As shown in FIG. 4, the DCH frame length (Tf) is 10 ms and the each frame consists of 15 time slots (slot#0˜slot#14); Accordingly, the length of one time slot (Tslot) is 2560 chips. Also, the DCH is related with alternately repeating dedicated physical data channels (DPDCHs) and dedicated physical control channels (DPCCHs). The DPCCH can include a TPC field (NTPC bit), TFCI field (NTFCI), and pilot field. The TFCI field includes the present channel information. For example, the TFCI field can indicate the data length and a coding type of the present frame.
Through the DCH and DSCH, the user's data for one user are transmitted at the same times a TFCI information (TFCI1) about the DCH and another TFCI information (TFCI2) about the DSCH are transmitted in the TFCI field at the same time. For this purpose, a TFCI field in one time slot can be divided into two halves for the TFCI1 and TFCI2.
There are two methods for transmitting the TFCI1 and TFCI2. The first method is transmitting the TFCI1 and TFCI2 in one code word on the basis of second order Reed Muller coding. This method is called Logical Split Mode.
The second method is generating two code words for respective TFCI1 and TFCI2 on the basis of a first order Reed Muller coding and transmit the two code words after mixing them in bit. This method is called Hard Split Mode. In case where the DCH is transmitted by the base stations in different radio network controllers, the second method is used for transmitting the TFCI2. In this case, the TFCI2 can be transmitted in some part of the whole wireless link. That is, the TFCI2 cannot be transmitted in the DCH of which radio network controller differ from the radio network controller that transmits the DSCH. Accordingly, in the Hard Split Mode, preferably different power controls should be used for TFCI1 and TFCI2, and also to control the DSCH power.
Typically, the DCH supports the soft handover, while the DSCH cannot support the soft handover. In the event that the DCH is in the soft handover state and the DSCH is transmitted from just one base station, it is required to perform different power controls to the DCH and DSCH. That is, the DCH generates TPC signal by summing the power from the base station, however, the DSCH power control is impossible through the TPC signal because the DSCH is transmitted from just one base station. For this reason, a particular power control different from the conventional one is required for the DSCH.
There are two methods for controlling the DSCH power control. The first is operating the SSDT (site selection diversity transmit) only in the uplink. When the mobile station performs soft handover, the mobile station measures the powers from every base station using the SSDT to select one base station that transmits the strongest power as a primary base station, and responsively transmitting to the RNC through a physical signaling. In this case, only the primary base station continues to transmit information and the non-primary base stations stop the transmissions. The operation in uplink means that the primary base station selection signal is transmitted in uplink, but there is no power on/off operation in downlink.
In this case, the DSCH power control can be performed in two modes. When the base station transmitting the present DSCH is the primary base station, the DSCH is transmitted in a little stronger power than the standard power. This power can be varied according to the TPC generated according to the DCH. On the other hand, if the base station is non-primary, then much higher power offset can be assigned. The power offset value can be highly set in order to receive the whole area of a cell.
In the second method, the mobile station generates TPC signals for both the DCH and DSCH and sends them to the base station. However, in the second method there is a problem in which the mobile station must measure the DSCH power as well as the DCH.
Now, the operation of power control in the downlink will be described in detail. Firstly, the mobile station measures a signal to interference ratio (SIR) of the DCH and compares the measured signal to the interference ratio (SIRest) to a target signal to ratio (SIRtarget). If SIRest is greater than SIRtarget, the mobile station transmits TPC signal of ‘0,’ to the base station. On the other hand, if SIRest is less than SIRtarget, the mobile station transmits TPC signal of ‘1.’ Thus, the base station adjusts the DCH power on the basis of received TPC signal according to equation 1.P(k)=P(k−1)+Ptpc(k)  Equation 1
The present DCH power is obtained by subtracting or adding the power (Ptpc(k)) adjusted by the TPC signal from/to the previous power P(k−1). That is, Ptpc(k) is +ΔTPC when TPCest(k) is 1, and Ptpc(k) is −ΔTPC when when TPCest(k) is 0. That is, the DCH power is increased as much as ΔTPC when the measured SIRest is less than the SIRtarget, the DCH power is decreased as much as ΔTPC when the measured SIRest is greater than the SIRtarget.
The power of TFCI field of DPCCH on the basis of the present DCH power P(k) cab be expressed as equation 2.PTFCI(k)=P(k)+PO1  Equation 2
where, the PO1 is a power offset between DPDCH and TFCI field. That is, the power of TFCI of the DPCCH is obtained by adding the power offset PO1 to the present DCH power. Accordingly, the conventional DSCH power control method can be adapted to the TFCI2 power control. The TFCI field belongs to the DPCCH such that the power control is performed in same manner.
As described above, the TFCI field can be TFCI1 and TFCI2. However, in DSCH split mode TFCI performance can be degraded because the TFCI2(TFCI of the DSCH) may not be transmitted from all of the base stations. In other words, since the TFCI field contains information about the number of data bits and coding method of the present frame, the data in the wireless frame cannot be detected if the TFCI field is not accurately received. In this case, the information on a spreading factor or data length cannot be transmitted.
On the other hand, during the soft handover of the mobile station, the power control is performed on the basis of the sum of power from all of the base stations that consist an active set. But, the TFCI2 is not transmitted from all of the base station but only from some of them. Accordingly, it is difficult to adjust the TFCI2 power to maintain at the predetermined quality.
In the conventional power control system, the power offset of the TFCI field can be performed only in the radio link setup and the DPCH power control is performed on the basis of the preset power offset. That is, since the power offset of the TFCI field for the DPCH is adjusted, it is impossible to assign another power if the channel environment or the active set topology has been changed.
Furthermore, even if strong power has been assigned to maintain the quality of the TFCI field, this results in waste of power since it does not adjust but fix the power level of the TFCI.
Now, a method for signaling the power offset obtained as above.
The communication protocols can be classified into control plane (see FIG. 5) and user plane (see FIG. 6) protocols because typically the control signaling for the system control and the end-user data are transmitted in different channels. The control plane protocol is a protocol being used in the wireless among the UTMS protocols. As shown in FIG. 5, the control plane protocol consists of radio resource control (RRC), radio access network application part (RANAP), radio network subsystem application part (RNSAP), and node B application part (NBAP).
In FIG. 5, the RRC is used between the mobile station UE and RNC, the NBAP is used between the base station Node B and the RNC as a lub interface protocol, the RNSAP is used between RNCs as a lur interface protocol, and the RANAP is used between the RNC and CN as an lu interface protocol. These radio network control plane protocols exist under a client-server environment. At the lu interface, the UTRAN acts as a radio access server and the CN acts as a client requiring access services to the UTRAN. Also, at the lub interface the base station and the RNC are a server and client, respectively, and at the lur interface the DRNC and the SRNC respectively act as a server and client. These protocols can include various control messages for the radio access bearer resources in the service areas between the base stations and RNCs, and between CN and RNCs.
Among the user plane protocols is a frame protocol (FP) for carrying UMITS user data frame. As shown in FIG. 6, the FP consists of lub FP, lur FP, and lu user plane protocol (lu UP) using at respective interfaces. These protocols perform various control function and uplink and downlink data transmission. For example, among these functions are timing adjustment and synchronization as in the asynchronous European CDMA. Also, these protocols have a function of transmitting the outer loop power control command to the mobile station.
FIG. 7 shows control frames used in the user plane protocol for DCH in the 3GPP lur/lub interface. As shown in FIG 7, there are ten kinds of control frames for outer loop power control, timing adjustment, DL synchronization, UL synchronization, DL signaling for DSCH, DL node synchronization, UL node synchronization, Rx timing deviation, radio interface parameter update, and timing advance. And each control frame is distinguished 8 bit coding information. Among the control frames the radio interface parameter update is used for updating the 8 bit connection frame number (CFN), 5 bit transmit power control (TPC) power offset, and 1 bit downlink power control (DPC) mode information (see FIG. 8). Also, the control frame format comprises 4 byte of payload.
The signaling using the control frame in the user plane has some advantages as the reaction is quicker than the signaling in the control plane and the size of the message is smaller. However, the signaling using the control frame in the user plane is unreliable. The control information transmitted from the control plane is called “control message,” and the control information transmitted from the user plane is called “control frame.”
FIG. 9a to FIG. 9d are block diagrams illustrating channel connection states between the base stations and the mobile station during the DSCH hard handover or an associated soft handover of DCH when the mobile station moves into a service area of new RNC. FIG. 9a shows a channel state before the soft handover of DCH associated with the DSCH, and FIG. 9b shows the channel state during the soft handover of DCH associated with the DSCH and before the DSCH hard handover. FIG. 9c shows the DSCH hard handover, and FIG. 9d shows the channel state after the soft handover of DCH associated with the DSCH.
FIG. 10a and FIG. 10b are drawings of the conventional signaling procedures during in which the associated DCH soft handover performs according to the movement of the mobile station (UE) without the DSCH hard handover as in the FIG. 9a and FIG. 9b. 
In the conventional signaling procedure, the power control to the TFCI in DSCH hard split mode, Is not independently performed but the TFCI power offset on the basis of NBAP and RNSAP of control plane of initial radio link setup is always used regardless of the movement of the mobile station or the number of radio links for transmitting the TFCI2.
Even though there have been many suggestions for performing TFCI power control in the DSCH hard split mode, any effective method compatible to the radio access network (RAN) interface standard of 3GPP have not been developed yet.
Also, for the TFCI power control in the DSCH hard split mode, it is required to use the message transmission method between the Node B and RNC and between RNCs as described above. Unfortunately, there is no guideline about power controlling message to the TFCI in the DSCH hard split mode and an operation procedure thereof. As a result, there are confusions in developing 3GPP asynchronous system and terminal using the TFCI power control in the DSCH hard split mode based on conventional technology.